Just an update - so it appears it happens intermittently, and to either one or all the fans regardless of which header it's plugged into.

The only 'change' since this started happening was a BIOS update to the latest version.

I've loaded the optimized defaults in the BIOS, checked/changed fan profiles etc but it still occurs intermittently.

What i have noticed is that usually it occurs during the first boot of the day. When starting up the fans spin and then after the 'Gigabyte logo' screen, that's when it'll stop, and stay stopped until (usually) i've logged into windows and can see my desktop.

Which makes me believe it may be BIOS related?